Photo detect element and an optical semiconductor device

ABSTRACT

An optical semiconductor device of the present invention is equipped with a photo detect element  10  comprising a photo detect part  7  provided with two photodiodes having two photodiodes having peak wavelength sensitivity in a visible light region and an infrared region, respectively and a amplifying operation processing circuit  8  for amplifying and processing outputs of the photodiodes, and characterized in that substrate resistivity R is as follows:  
     1≦ R ≦3(Ω cm )

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Applications No. 2002-97046, filed onMar. 29, 2002: the entire contents of which are incorporated herein byreference.

BACKGROUND OF TIE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a photo detect element and anoptical semiconductor device and, more particularly, to an opticalsemiconductor device that is used in so-called visual sensitivity andluminous intensity measurement etc. for measuring luminous intensity bysensing visible light only.

[0004] 2. Description of Related Art

[0005] In recent years, in optical semiconductor devices that are usedfor visual sensitivity and luminous intensity measurement, a singlechipped photo detect element comprising photo diodes (hereafter,referred to as PD) and an amplifying operation processing circuit toperform the amplification and processing of optical signals from PD areused. As shown in FIG. 1, an n-type region 2 is formed on the surface ofa p-type silicon substrate 1 and a p-type region 3 is formed on thesurface of this n-type region 2. At two PN junctions formed in thevertical direction, two PD (PD₁ 4, PD₂ 5) are formed.

[0006] An optical absorption rate on Si wafer varies depending on adepth from the surface and this optical absorption depth dependencydiffers depending on a wavelength, as shown FIG. 9. As the distances ofabove-mentioned two PN junctions from the surface of the substrate aredifferent, the spectral response characteristic differs for each PD, forexample, as shown in FIG. 3. The photodiode PD1 has the maximumsensitivity at around a wavelength 600 nm and the photodiode PD₂ at theposition deeper than the photodiode PD₁ has the maximum sensitivity ataround a wavelength 900 nm in FIG. 3.

[0007] Thus, a predetermined sensibility; that is, visual sensitivitycan be obtained by a photo detect element by processing outputs from thediodes PD₁ and PD₂ that have different spectral response characteristicsthrough the amplifying operation processing circuit. The luminousintensity corresponding to the spectral luminous efficiency is measuredby eliminating the infrared component of the output of the photodiodePD₁ using the infrared component of the output of the PD₂.

[0008] A photo detect element 10 having such characteristic as this ismounted on a glass epoxy resin (hereinafter referred to as Glass-Epoxy)made substrate and the like, cured and bonded thereon, as shown in FIGS.10A-10D, and FIG. 11. After bonding the terminals of the photo detectelement with gold wires, an enclosure is formed and separated aftertransfer molded by a transparent epoxy resin 14 in order to protect thephoto detect element 10. Then, the photo detect element 10 is connectedto an external circuit in a metallized portion 16.

BRIEF SUMMARY OF THE INVENTION

[0009] In such the optical semiconductor device, carriers are generatedby infrared ray incoming from the side of the photo semiconductor unitand a dicing face of the photo detect element, infrared ray near 1000 nmturned around in the photo detect element. As a result, the photo detectelement gets a sub-peak of unnecessary infrared component of 900˜1200 nm(centering around 1000 nm) in addition to a peak of visual sensitivity(550 nm) as shown in FIG. 8. That is, as a conventional opticalsemiconductor device detects unnecessary infrared component, theillumination intensity corresponding to spectral luminance efficiencycould not be made.

[0010] That is, as an optical semiconductor device used for detectingspectral luminance efficiency, a defect is produced as a peak isdetected in the infrared region. For example, when two light sources areset at the same luminous intensity, there is originally no difference inoptical output current values and a light source ratio (A lightsource/fluorescent lamp) is one time. However, an optical semiconductordevice having a characteristic to sense a sub-peak of a visualsensitivity in the above-mentioned infrared region senses the infraredregion and when components of the infrared region of A light source aresensed, an output current value increases and a light source ratiobecomes worse. Further, when flat and reflective materials (for example,white glass epoxy resin plate, etc.) are used for a substrate to installa photo detect element, the infrared component region is also reflectedby the reflection of light from the substrate and the light source ratiois made further worse.

[0011] As explained in the above, a conventional optical semiconductordevice has a sensing character in the infrared region and therefore,there was such a problem that the output also depends on infrared raywhen a optical semiconductor device is used for visual sensing ofluminous intensity.

[0012] Therefore, an object of the present invention is to provide anoptical semiconductor device with its spectral sensitivitycharacteristic controlled more precisely by suppressing the sensitivityin an infrared region with such a defect of a conventional opticalsemiconductor device removed.

[0013] The optical semiconductor device of the present invention ischaracterized in that an n-type region is formed on a p-type substrate,a p-type region is formed on the surface of this n-type region, a photodetect element equipped with a first photo diode having peak wavelengthsensitivity in a visible spectral region is provided on the interfacebetween the n-type region and the p-type region, a second photo diodehaving peak wavelength sensitivity in the infrared region is provided onthe interface between the p-type substrate and the n-type region, andspecific resistance R of the p-type substrate is 1≦R≦3(Ωcm).

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a cross-sectional view showing PD₁, PD₂ used in enembodiment of the present invention;

[0015]FIG. 2 is a circuit diagram of an photo detect element used in anembodiment of the present invention;

[0016]FIG. 3 is a diagram showing the spectral sensitivitycharacteristics of PD₁, and PD₂ used in an embodiment of the presentinvention;

[0017]FIG. 4 is a top view of an example of a photo detect element usedin an embodiment of the present invention viewed from the above;

[0018]FIG. 5A˜FIG. 5D are diagrams showing the construction of a optsemiconductor device in an embodiment of an present invention;

[0019]FIG. 6A and FIG. 6B are a plan view and a cross-sectional view ofan optical semiconductor device in an embodiment of the presentinvention.

[0020]FIG. 7A and FIG. 7B are diagrams showing the spectral sensitivitycharacteristic of an optical semiconductor device in an embodiment ofthe present invention;

[0021]FIG. 8 is a diagram showing the spectral sensitivitycharacteristic of a conventional optical semiconductor device;

[0022]FIG. 9 is a diagram showing the depth dependency of opticalabsorption in Si wafer;

[0023]FIG. 10A˜FIG. 10B are diagrams showing a conventional opticalsemiconductor device;

[0024]FIG. 11 is a diagram showing a conventional optical semiconductordevice; and

[0025]FIG. 12 is a diagram showing a spectral sensitivity characteristicof the photodiodes PD₁,PD₂ when a filter provided in the photoacceptance portion is in the thin film laminated structure in anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] An embodiment of the present invention will be described below indetail referring to FIG. 1 through FIG. 8.

[0027] As shown in FIG. 1, an n-type region 2 and a p-type region 3 areformed on the surface of a p-type silicon substrate 1 and a photoacceptance portion having PD₁ 4 and PD₂ 5 is formed. Further, a filter 6is formed on the surface. An amplifying and an arithmetic circuit isformed with this photo acceptance portion. As shown in FIG. 2, in thisoperational amplifier, the PD₁ and PD₂ of the photo detect elementconvert photo signals into electric signals Ip₁, Ip₂, which are thenoutput through an initial stage amplifier, a differential circuit, anarithmetic circuit and an amplifier. Further, the spectral sensitivitycharacteristic of PD₁ and PD₂ is the same as before as shown in FIG. 3.

[0028] Then, a photo detect element 10 is formed by arranging a photodetect part 7, an amplifying operation processing circuit 8, a bondingpad 9 and diced as shown in FIG. 4. Then, the photo detect element 10 ismounted on a mount bed 12 and terminals of the photo detect element 10are bonded to a metallic lead frame 11 with a gold wire 13 shown in anexternal view in FIG. 5A and FIG. 5B, a plan view and a cross-sectionalview in FIG. 6A and FIG. 6B. Thereafter, an enclosure is molded andformed by an transparent epoxy resin 14 to protect the photo detectelement 10 and an optical semiconductor device is formed and connectedto an external circuit at the outer lead portion of the exposed metalliclead frame 11.

[0029] In such the optical semiconductor device, when the output currentby PD₁, PD₂ are Ip₁, Ip₂ and processed through the amplifying operationprocessing circuit, the entire output current Iout becomes as shownbelow:

Iout=m×Ip ₁ −n×(Ip ₁ +Ip ₂)

[0030] That is,

Iout=β(Ip ₁ −αIp ₂)

[0031] α: Coefficient of subtraction factor (n/(m−n))

[0032] β: Coefficient of amplification (m−n) Further, coefficient ofamplification β is a value set by a demanded output current value. Aphoto detect space S of the photo detect part is 0.1225 mm² (0.35 mmsquare).

[0033] With regard to Package A of an optical semiconductor device thatis formed as described above and Package B that is formed with a similarphoto detect element mounted on a glass epoxy substrate and transfermolded, a luminous source ratio when a coefficient of subtraction factorand a resistivity of a p-type silicon substrate were changed and peakrelative strength of infrared component to peak strength are obtained asshown in Table 1. TABLE 1 luminous Resistivity source infarred # Package(Ω cm) α ratio component 1 A 2 ˜ 3 0.3   1 ˜ 1.5  5% 2 0.55 0.9 ˜ 1.2 5% 3 1 2 ˜ 4 10% 4 4 ˜ 6 0.3 1.5 ˜ 2   20% 5 B 2 ˜ 3 0.3 1.5 ˜ 2   30%6 1 2 ˜ 4 30% 7 4 ˜ 6 0.3 2 ˜ 3 50% 8 1 3 ˜ 6 50%

[0034] So far, a coefficient of subtraction magnification a was nottaken into consideration. That is, α=1 and it was seen that it becomespossible to reduce a luminous source ratio and infrared component whenthe coefficient of subtraction magnification a is set in the followingrange.

0.3≦α≦0.55

[0035] Further, the range of this coefficient of subtractionmagnification αis especially effective when the photo detecting space Cis below 0.25 and is

0.0765≦S≦0.1225(mm ²)

[0036] Further, in an effort to improve reliability of an opticalsemiconductor device and extend its life, a p-type substrate having ahigh resistivity of 4˜6 Ωcm was so far used for substrate of photodetect element. However, it is seen that it becomes possible to reduce alight source ratio and infrared component by controlling the number oflattice defects of p-type substrate and using substrate having a lowresistivity of 2˜3 Ωcm inversely in this embodiment. This is consideredbecause when substrate resistivity R becomes low, spectral sensitivitycharacteristic shifts to higher energy of photon than absorption bandgap, that is, to the short wavelength side at lower substrateresistivity R.

[0037] Further, the more low the substrate resistivity R is, the morepreferable in order to reduce light source ratio and infrared component.However, when substrate reisistivity R is less than 1 Ωcm, it does notfunction as PD. On the other hand, over 3 Ωcm, a sufficient effectcannot be obtained and 1˜3 Ωcm is adequate. Further, a desired value ofsubstrate resistivity can be obtained by adjusting the doping amount ofp-type impurity.

[0038] Further, with regard to an optical semiconductor device similarlyformed, a light source ratio and relative strength of infrared componentwhen a chip thickness dc of a photo detect element shown in FIG. 5D wereobtained as shown in Table 2. Further, a package thickness dp of anoptical semiconductor device is 0.7 mm and a metallic lead frame in0.1˜0.15 mm thick is used. TABLE 2 chip luminous Resistivity thicknesssource infrared # pacage (Ω cm) α dp (mm) ratio component  9 A 2 ˜ 30.55 0.1 0.85 ˜ 1.1   3% 10 0.15 0.9 ˜ 1.2  5% 11 0.2   1 ˜ 1.5  8% 120.3   1 ˜ 1.5 10% 13 B 0.3 0.3 1.5 ˜ 2   30%

[0039] So far, the chip thickness dp of the photo detect element wasmade to 0.3 mm by lapping. Chip thickness dc/package thickness dp (chipthickness ratio) was 40˜55% for the package thickness dp about 0.55˜0.7mm of an optical semiconductor device. However, it is seen that slightsource ratio and infrared component can be reduced by reducing the chipthickness dc to 0.2 mm or below (0.05 mm or more is realistic). The chipthickness ratio (dc/dp) below 0.25 is good (0.07 or more is realistic)and 0.2 or below is preferable.

[0040] When the chip thickness ratio (dc/dp) is made to 0.25 or below,generation of crack by coefficient of expansion of the transparent epoxyresin 14, the photo detect element 10 and the metallic lead frame 11 canbe prevented.

[0041] Further, with regard to an optical semiconductor device (PackageA) that is similarly formed, light source ratio and relative strength ofinfrared component when a mount bed size of a metallic lead frame onwhich the photo detect element is installed were changed are obtainedwith the results as shown in Table 3.

[0042] Here, the mount bed is a portion of the lead frame on which aphoto detect element is mounted. As shown in FIG. 6A and FIG. 6B that isequivalent to the cross sectional diagram of Section A-A in FIG. 6A,what is equivalent to the length of one side is substantially a mountbed size tm. Further, when a chip size is tc, Δt is given by thefollowing formula:

Δt=(tm−tc)/2

[0043] Δt is equivalent to a distance between the edge of the mount bed12 when the photo detect element 10 is mounted at the center of themount bed 12 and the edge of the photo detect element 10. Further, whenΔt differs depending on a position, its maximum value is made as Δt. Thechip size tc is 1.1 mm. TABLE 1 luminous infrared # tm (mm) Δt (mm)source ratio component 14 1.1 0 0.9 ˜ 1.3  3% 15 1.2 0.05 0.9 ˜ 1.3  5%16 1.3 0.1   1 ˜ 1.5  8% 17 1.4 0.15 2 ˜ 3 10% 18 1.5 0.2 2 ˜ 3 12%

[0044] Thus, it can be seen that light source ratio and infraredcomponent can be suppressed by reducing Δt to below 0.1 mm. This isbecause the reflection of light including infrared component from themount bed is suppressed, and it is ideal to make the chip size tc equalto the mount bed size tm. Further, it is preferred not to mount anythingreflecting light in a range within 0.1 mm around the mount bed 12.

[0045] Further, the mount bed 12 is made here as the photo detectelement mounting part of the metallic lead frame 11. This is also thesame on ball Grid Array (BGA) substrate, etc. and it is effective tosuppress the distance from the photo detect element to the mount bededge similarly. At this time, it is necessary to make regions other thanthe mount bed 12 of the substrate using material and color (black) notreflecting light.

[0046] Further, in this embodiment, a case wherein the chip size tc isbelow 1.1 mm is described. However, even when it is below 1.1 mm, if Δtis below 0.1 mm, it is possible to reduce infrared componentsufficiently. However, when Δt=0.1 mm or below, it is needless to saythat infrared component can be reduced relatively in a large chip size.

[0047] Further, the spectral responsibility was measured on opticalsemiconductor devices formed as described above with the results shownin FIG. 7A and FIG. 7B. Further, FIG. 7A is a spectral sensitivitycharacteristic diagram showing relative sensitivity of Package B with alow substrate resistivity R (2˜3 Ωcm) and an optimized coefficient ofsubtraction magnification α to wavelengths. FIG. 7B is a spectralsensitivity characteristic diagram showing relative sensitivity of animproved Package A with optimized chip thickness ratio and Δt.

[0048] It is seen that the spectral sensitivity characteristic shown inFIG. 7A and FIG. 7B is sharply improved when compared with the spectralsensitivity characteristic of a conventional optical semiconductordevice (Package B of α=1, high substrate resistivity). In particular, agood spectral sensitivity characteristic with almost less sensitivity inthe infrared region is obtained in FIG. 7B. Also in FIG. 7A, a goodspectral sensitivity characteristic was obtained though certainsensitivity was left in the infrared region and it is seen that theoptimization of parameters is also effective in Package B.

[0049] In addition, in Package B, a printing substrate needs to make itthe quality of the material which does not reflect light, and a color(black).

[0050] Further, in the construction diagram of the photo detect partshown in FIG. 1, when the filter 6 having the characteristic to shut offinfrared components is used, the sensitivity in the infrared region canbe further reduced and an optical semiconductor device having more highprecisely controlled spectral sensitivity characteristic can beobtained.

[0051] The filter 6 is formed by laminating, for example, a thin film oftitanium dioxide (TiO₂) having a high refractive index and a thin filmof silicon dioxide (SiO₂) alternately. The thin film titanium dioxideand thin film silicon dioxide are, for example, 0.24 μm thick andlaminated by, for example, deposition for every 75 layers to a filter oftotal 150 layers and total thickness 36 μm.

[0052] Deposition conditions when laminating titanium dioxide andsilicon dioxide by deposing are, for example, degree of vacuum is 1Pa˜2×10⁻⁴ Pa, substrate temperature 120˜350° C., and resistance heatingor an electron gun is used.

[0053] Further, the total number of films that are laminated for thefilter 6 is selected in a range of about 50˜150 layers. A filter in thislaminated structure is formed by deposition directly on the photo detectpart.

[0054] Spectral sensitivity characteristic by photodiodes PD₁ and PD₂when the photo detect part has a filter in the laminated structureformed as described above is shown in FIG. 12. The characteristic of thephotodiode PD₁ is B₁ and the characteristic of the photodiode PD2 is B₂.When these characteristics B₁, B₂ are compared with the characteristicshown in FIG. 3, it is seen that when a filter in the laminatedstructure is used, the wavelength is around 650 nm or above and spectralsensitivity was lowered and spectral sensitivity at a wavelength above800 nm is nearly zero and almost constant.

[0055] As described above, when a filter with thin films havingdifferent refraction factors laminated alternately is directly providedto the photo detect part, a further good spectral sensitivitycharacteristic can be obtained irrespective of incident range of light.

[0056] When this filter in laminated structure is provided to the photodetect part by the deposition, there is the less possibility ofseparation by heat generated when soldered or secular change of thefixed portion even when the filter is mounted on the surface of a wiringsubstrate, etc. by the reflow soldering and a highly reliable opticalsemiconductor device is obtained.

[0057] According to the present invention, it is possible to provide anoptical semiconductor device having the suppressed sensitivity in theinfrared region and a high precisely controlled spectral sensitivitycharacter.

What is claimed is:
 1. A photo detect element comprising: a firstphotodiode having peak wavelength sensitivity in a visible light region;a second photodiode having peak wavelength sensitivity in an infraredregion; wherein said first photodiode comprises a p-type substrate and an-type region on said p-type substrate, and said second photodiodecomprises said n-type region and said p-type region on said n-typeregion, and resistivity R of said p-type substrate is as follow:1≦R≦3(Ωcm)
 2. A photo detect element according to claim 1, wherein whena thickness of said optical semiconductor device is assumed to be dc,dc≦0.2 mm is satisfied.
 3. An optical semiconductor device comprising: aphoto detect element equipped with a first photodiode having peak wavelength sensitivity in a visible light region and a second photodiodehaving peak wavelength sensitivity in an infrared region; a packagecovering said photo detect element; wherein when a thickness of saidpackage is assumed to be dp, dc/dp≦0.25 is satisfied.
 4. An opticalsemiconductor device comprising: a photo detect element equipped with afirst photodiode having peak wave length sensitivity in a visible lightregion and a second photodiode having peak wavelength sensitivity in aninfrared region; a mount bed with a transparent resin portion coveringthe photo detect element and the photo detect element mounted thereon;and a means for connecting to an external circuit, and when the mountbed size is tm and the size of the photo detect element is tc and whenΔt=(tm−tc)/2 O≦Δt≦0.1(mm) is satisfied.
 5. An optical semiconductordevice comprising: a photo detect element equipped with a firstphotodiode having peak wavelength sensitivity in a visible light regionand a second photodiode having peak wavelength sensitivity in aninfrared region; and an amplifying operation processing circuit forprocessing and outputting an output current Ip₁ from said firstphotodiode and an output current Ip₂ from said second photo diode; andwherein when α is assumed to be a coefficient of subtractionmagnification and β is assumed to be a coefficient of amplification, anoutput Iout from said amplifying operation processing circuit obtainedby the following formula, Iout=β(Ip ₁ −αIp ₂) 0.3≦α≦0.55
 6. An opticalsemiconductor device according to claims 3 or 4, wherein a space S of aphoto detecting part is as follows: S≦0.25 mm²
 7. An opticalsemiconductor device comprising: a photo detect element equipped with afirst photodiode having peak wave length sensitivity in a visible lightregion and a second photodiode having peak wavelength sensitivity in aninfrared region; a package covering said photo detect element; and afilter formed on said photo detect element by laminating plural kinds offilms having different refraction factors by deposition, wherein saidfirst photodiode comprises a p-type substrate and a n-type region onsaid p-type substrate, and said second photodiode comprises said n-typeregion and said p-type region on said n-type region, and a resistivity Rof said p-type substrate is as follows: 1≦R≦3(Ωcm)
 8. An opticalsemiconductor device according to claim 7, wherein dc≦0.2 mm when athickness of a photo detect element is dc.
 9. An optical semiconductordevice according to claim 8, wherein the following formula is satisfiedwhen a thickness of the package is dp: dc/dp≦0.25
 10. An opticalsemiconductor device according to claim 9, wherein the filter is formedby laminating titanioum dioxide (TiO₂) thin films and silicon dioxide(SiO₂) thin films alternately.
 11. An optical semiconductor devicecomprising: a photo detect element equipped with a first photodiodehaving peak wavelength sensitivity in a visible light region and asecond photodiode having peak wavelength sensitivity in an infraredregion; and an amplifying operation processing circuit for processingand outputting an output current Ip₁ from said first photodiode and anoutput current Ip₂ from said second photodiode; a filter formed on saidphoto detect element by laminating plural kinds of films havingdifferent refraction factors by deposition; and wherein when α isassumed to be a coefficient of subtraction magnification and β isassumed to be a coefficient of amplification is, an output Iout fromsaid amplifying operation processing circuit obtained by the followingformula, Iout=β(Ip ₁ −αIp ₂) 0.3≦α≦0.55
 12. An optical semiconductordevice according to claim 11, wherein said filter is formed bylaminating thin films of titanium dioxide (TiO₂) and silicon dioxide(SiO₂) alternately.